From NEC Retro
and then I realised the PC-9821 line and the PC98-NX line are two separate things.
I can't find a list of PC-9821 games. Better make one:
(if it says PC-9801 it is a PC-9801 game. If it says DOS or Windows it is a DOS or Windows game. If it says specifically PC-9821 and nothing else, it's a PC-9821 game)